Anthropogenic nitrogen (N) deposition in urban areas has the potential to alter plant growth, biomass allocation, tissue quality, and phenology. The combustion of fossil fuels and application of fertilizers contributes to N-laden precipitation in the form of nitrate (NO3-) and ammonium (NH4+) ions, respectively. Native plants in nutrient poor areas are particularly susceptible to an altered N cycle via direct toxicity and interspecies competition. Studying the effects of these N additions on native plants can help us understand the impact of alterations to the N cycle in urban ecosystems. We examined the relationship between deposition rate and growth of the dominant native prairie grass, little bluestem (Schizachyrium scoparium), by conducting a phytometer experiment at six sites with varying levels of urban development in North Texas. Percent urban development was calculated using a 10k radius around each site. Grass tiller count and length were measured monthly from April-June 2014. Timing of first flowering was also tracked. After three months, the plants were harvested and separated into roots, vegetative tillers, and reproductive tillers to obtain dry biomass. For the duration of the experiment, bulk N deposition was collected at each site using ion exchange resins.
Results/Conclusions
The majority of N deposition in North Texas comes in the form of NH4+. For the three-month period, the average across all sites was 1.6 kg/ha/m2 (range 0.8-3.3 kg/ha/m2), while the average NO3- deposition was 0.621 kg/ha/m2 (range 0.576-0.649). There was a significant negative relationship between urban development and NH4+ deposition but no correlation with NO3-. Interestingly, total nitrogen deposition (NO3-N + NH4-N) at a site was significantly negatively correlated with the total biomass of little bluestem. This resulted from biomass of reproductive tillers and vegetative tillers being significantly negatively correlated with N deposition; root biomass did not differ among the sites. In addition, with greater N deposition, plants allocated a smaller proportion of aboveground resources to reproductive tillers. The plants at the two most rural sites started flowering two weeks later than the other sites, indicating the potential for restricted gene flow among populations. Our results suggest that the negative aboveground response in vegetative and reproductive tillers may lead to lower reproductive rates in urbanized areas. These effects on little bluestem, an integral part of tallgrass prairie communities, have consequences not only for the species, but for the preservation of remnant native prairies.